In the past procedures have been proposed for the conversion of fibers and films formed from acrylic polymers to a modified form processing enhanced thermal stability. Such modification has generally been accomplished by heating a fibrous material or film in an oxygen-containing atmosphere at a moderate temperature for an extended period of time.
U.S. Pat. Nos. 2,913,802 to Barnett, 3,285,696 to Tsunoda, and 3,539,295 to Ram disclose processes for the conversion of fibers of acrylonitrile homopolymers or copolymers to a heat resistant form. The stabilization of fibers of acrylonitrile homopolymers and copolymers in an oxygen-containing atmosphere involves (1) a chain scission and crosslinking reaction of adjoining molecules as well as (2) a cyclization reaction of pendant nitrile groups. It is generally recognized that the rate at which the stabilization reaction takes place increases with the temperature of the oxygen-containing atmosphere. However, the stabilization reaction must by necessity be conducted at relatively low temperatures (i.e. below about 300.degree. C.), since the cyclization reaction is exothermic in nature and must be controlled if the original fibrous configuration of the material undergoing stabilization is to be preserved. Accordingly the stabilization reaction tends to be time consuming, and economically demanding because of low productivity necessitated by the excessive time requirements. Prior processes which may shorten the period required by the stabilization reaction include those disclosed in U.S. Pat. Nos. 3,416,874, 3,592,595, 3,647,770, 3,650,668, 3,656,882, 3,656,883, 3,708,326, 3,729,549, 3,767,773, 3,813,219, 3,814,577, 3,820,951, 3,850,876, 3,917,776, 3,923,950, 3,961,888, 4,002,426, and 4,004,053; British Pat. Nos. 1,280,850 and 1,478,775; and Soviet Author's Certificate 389,012.
While stabilized acrylic fibrous materials may be used directly in applications where a non-burning fiber is required, demands for the same have been increasingly presented by manufacturers of carbonized fibrous materials. Carbonized fibrous materials are commonly formed by heating a stabilized acrylic fibrous material in a non-oxidizing atmosphere such as nitrogen or argon, at a more highly elevated temperature. During the carbonization reaction elements such as nitrogen, oxygen, and hydrogen are substantially expelled. Accordingly, the term "carbonized" as used in the art commonly designates a material consisting of at least about 90 percent by weight, and generally at least about 95 percent carbon by weight. Depending upon the conditions under which a carbonized fibrous material is processed, it may or may not contain graphitic carbon as determined by the characteristic x-ray diffraction pattern of graphite. See, for instance, commonly assigned U.S. Pat. Nos. 3,656,904, 3,723,605, 3,775,520, 3,900,556, and 3,954,950.
It is an object of the present invention to provide an improved process for forming thermally stabilized acrylic fibers and films.
It is an object of the present invention to provide an improved process for forming a thermally stabilized acrylic fibrous material or film which satisfactorily can be carried out on an accelerated basis and/or at a lower stabilization temperature.
It is an object of the present invention to provide an improved process for forming thermally stabilized acrylic fibers and films in which the undesirable exothermic nature of the stabilization reaction is controlled.
It is another object of the invention to provide an improved process for forming stabilized fibrous materials or films derived from acrylic polymers which results in a product which is suitable for carbonization, or carbonization and graphitization.
These and other objects, as well as the scope, nature, and utilization of the invention will be apparent from the following detailed description and appended claims.